Strain rate change tests were performed during the low-cycle fatigue of polycrystalline Cu by using the plastic strain as the control variable. The evolution of dislocation interactions was observed by determining the activation area and true stress as a function of the cumulative plastic strain. The activation areas at each of 3 plastic strain amplitudes (∆εp/2 = 0.2, 0.4 and 0.6%) had initial values of about 2000b2. These decreased to 600b2 during cyclic loading up to saturation. This suggested the occurrence of a transition from forest dislocation cutting to an increased contribution of cross-slip as the predominant rate-controlling mechanisms of dislocation motion. Haasen plots of the normalized inverse operational activation area (b2/∆a), for specimens cycled to saturation, exhibited a deviation from linearity which was similar to that observed for monotonic deformation. This non-linearity represented a failure of the Cottrell¯Stokes law that correlated the development of characteristic dislocation structures during cyclic deformation. Tests which were performed under various stresses at saturation revealed a linear dependence of b2/∆a upon the true stress. The athermal stress (σb = 86.5MPa), measured at saturation by extrapolating the activation area data, compared favorably with the value (σb = 80MPa) which was deduced from a Bauschinger analysis at a plastic strain amplitude of 0.6%. The athermal stress values also varied with plastic strain amplitude, as expected; resulting in a constant σb/σ value of about 0.5.

Evolution of Dislocation Glide Kinetics during Cyclic Deformation of Copper. G.C.Kaschner, J.C.Gibeling: Acta Materialia, 2002, 50[3], 653-62